I am not sure if this counts as a bug, but I have node/add aliased to another page to be able to have a customized content creation "dashboard." This has never given me problems so far but when I turned on sub-path alias, any actual node add pages (node/add/blog, for instance) just printed out my custom node/add page.

For now, I have it working by adding this to subpath_alias_url_inbound_alter():

    if (substr($path, 0, 8) == 'node/add') {
      return;
    }

Is this a flaw in the module or something outside your control? I can't really tell.